Method and a system for determining wheel imbalances of at least one wheel on a vehicle

ABSTRACT

A system and a method of determining imbalances of at least one wheel on a vehicle, when said wheel is rotating, is provided. The method includes the steps of providing a vibration signal from at least one wheel vibration sensor associated with said wheel; providing an angular velocity signal of the rotation of said wheel, the angular velocity signal including a reference signal indicating the start of a wheel revolution; and based thereupon performing signal processing upon these signals for detecting a periodic signal of a predetermined nature corresponding to imbalances in said wheel and determining the position upon said at least one wheel of such imbalance. Accordingly, a wheel imbalance detection system separate from the vehicle is no longer necessary, because the present method provides an indication as to the precise location and type of any detected imbalance in a wheel. By being able to locate the position upon the wheel of such wheel imbalance, the maintenance time used is reduced considerably. Further, the possibility of an early detection of a wheel imbalance reduces the risk for damages to develop further.

BACKGROUND AND SUMMARY

The present invention relates to a method of determining imbalances ofat least one wheel on a vehicle, when said wheel is rotating. Further,it relates to a system for performing said method.

When a vehicle with wheels is being driven, this exposes it to wear overtime, which may influence the performance of said vehicle. Steeringsystem, tire and wheel wear are continually resulting in frequentmaintenance repairs. If not detected and repaired, this wear may lead toincreased steering inaccuracies, and in the worst case scenario toaccidents and risk for damage to driver, vehicle, bystanders andmaterial.

Said wear induces vibrations in all three dimensions into the vehicleand its wheels, because different types of wheel imbalances develop dueto this wear over time. For the purpose of the present invention, theterm “wheel imbalances” comprises different types of wheel imbalances.The most common wheel imbalance type is known as an out of balancecondition, e.g. where a wheel is having a non circular wheel shape dueto uneven tire wear as is shown in FIG. 1 a, or an incorrect placing ofbalancing weights inside the wheel. There are two types of imbalances,namely static imbalances, which occurs when there is a heavy or lightspot in the tire so that the tire won't roll evenly and the tire/wheelassembly undergoes an up-and-down movement, and dynamic imbalances,which occurs when there is unequal weight on both sides of thetire/wheel assembly's circumferential centreline.

Other types of wheel imbalances comprises wheel run-out, such as radialwheel runout in an “out-of-round” situation where vibrations areproduced as the wheel spindle moves up and down, i.e. where a wheel hasits circular shape transformed into an elliptical one, e.g. by animpact, as shown in FIG. 1 b, and lateral run-out resulting in aside-to-side or wobbling movement of the tire and wheel, which is lesscommon than radial run-out. Sensitivity of a vehicle to vibration fromradial run-out is four to eight times that of wobble from lateralrun-out.

Other types of wheel imbalances comprises when a wheel 20 has aneccentric rotational axis 20 c relative to the wheel axle axis, as shownin FIG. 1 c, and/or a wheel 20 is supported by a suspension 22, whichfunction is impaired, as shown in FIG. 1 d. Imbalances can alsooriginate from defects in the steering system of the vehicle.

When a wheel is provided with a wheel imbalance, the rotation of saidwheel upon a surface G or even independently from any surface impartswheel vibrations, all of which may be more or less detectable in alldirections x, y, and z as indicated with arrows in the FIGS. 1 a to 1 d.The surface G may be the ground surface upon which a vehicle is runningor alternatively a roller provided test surface, or the axle may belifted up from the ground, whereupon the acceleration corresponds to aspecific mass when the wheel is spinning at a given rate, whereupon anyimbalance type is detectable.

Prior art systems for detecting wheel imbalances have been disclosed,both conventional systems comprising a separate system from the vehicle,where the vehicle is at holding still and the wheels are turning, andalso systems for a vehicle being driven

These prior art systems comprise the system disclosed in EP 0 421 065comprising accelerometers along an x, y and z direction and wheel speedindicators for each wheel and an on board display indicating which typeof wheel imbalance is detected after performing frequency analysis ofthe measurements from the accelerometer and wheel speed indicator.

In U.S. Pat. No. 6,353,384 is disclosed another wheel imbalancedetection system and method for a vehicle while driving for determiningan out of balance condition in a wheel, comprising a singleaccelerometer provided upon an axle mounting two wheels, where thecombined wheel vibrations from these two wheels and wheel speeds from aconventional ABS-system is used for said determination. With thissolution, it is not possible to detect from which wheel the imbalanceoriginates.

However, such prior art systems are not able to indicate where upon thewheel, such an imbalance is positioned. This is a disadvantage, becausethis requires the use of two systems, that is an on board systemindicating that a wheel imbalance is in fact present and what type ofwheel imbalance it is, and a more sensitive separate system, e.g. amaintenance apparatus for a precise location of said wheel imbalance,which accordingly increases the costs of installation and maintenance ofboth systems. These systems also require that each wheel is dismountedfrom the vehicle for the analysis.

This is especially a problem for large vehicles, wherein often more thanfour wheels are provided, because if an on board prior art system e.g.such as disclosed in U.S. Pat. No. 6,353,384, indicates an imbalance inone of the wheels on an axis, several wheels must be checked, increasingthe time used in trying to locate such a wheel imbalance. The wheels ofa heavy vehicle are heavy and difficult to handle, which increases thecost for the examination.

On this background, it is desirable to provide a method and a system fordetermining imbalances of at least one wheel on a vehicle, whichalleviate the above mentioned disadvantages, and provide a positiveidentification of where upon said wheel, such imbalance is located inorder to ease the maintenance repair of said wheel when needed.

According to aspects of the present invention, a method and a system forperforming said method for determining imbalances of at least one wheelon a vehicle, when said wheel is rotating, are provided. The methodcomprises the steps of: providing a vibration signal from at least onewheel vibration sensor associated with said wheel, said vibration signalcomprising at least vertical acceleration along a y-direction; providingangular velocity signals of the rotation of said wheel comprising areference signal indicating the start of a wheel revolution; basedthereupon performing signal processing upon these signals for detectinga periodic signal of a predetermined nature corresponding to imbalancesin said wheel and determining the position upon said at least one wheelof such imbalance; and indicating the position of such imbalance in thewheel, and optionally other wheel imbalance characteristics, such asimbalance type.

Accordingly, a wheel imbalance detection system separate from thevehicle is no longer necessary, because the present method provides anindication as to the precise location and type of any detected imbalancein a wheel. Based on the indication of imbalance location on each wheelprovided with such system, any type of wheel imbalance may be locatedand quickly attended to by turning the wheel into position, and inspectand repair the point of the wheel imbalance. By being able to locate theposition upon the wheel of such wheel imbalance, the maintenance timeused is reduced considerably. Further, the possibility of an earlydetection of a wheel imbalance reduces the risk for a small damage tothe wheel condition to develop further, such as a wear induced zone ofbreakage.

In a preferred embodiment of the method according to the invention, saidreference signal is provided by a predetermined number of pulses,wherein one is selected for an indication of the start of a wheelrevolution. Thus, a reference signal is provided for an accuratedetermination of wheel revolution start.

In another preferred embodiment of the method according to theinvention, said reference signal is provided by an ABS sensor providingone signal pulse per revolution, which is different from the othersignal pulses. This may e.g. be a pulse with a shorter or longer pulsewidth than the other ABS-pulses in one wheel revolution, correspondingto an ABS sensor being provided with a broader or shorter tooth than theother teeth, which effectively identifies the wheel revolution start asa reference signal.

In another preferred embodiment of the method according to theinvention, the method further comprises the step of indicating thenumber, weight and position of counter balancing weights required tobalance the wheel, when the wheel imbalance type is determined to be anout of balance type. Thus, a supplemental use of a balancing apparatusis not necessary any more, as a fully operational out of balancedetection and balancing method is available by this method.

In a preferred embodiment of the system according to the presentinvention, said wheel vibration measurement means comprise one or moreone-, two, or three-dimensional accelerometers, which are provided on anon-rotating end section of the axle mounting said at least one wheel,adjacent to said wheel. By being provided on a non-rotating part of theaxle, e.g. in the wheel hub or on the axle end section adjacent to thewheel, interfering rotational vibrations in either direction is avoided.Accelerometers provide accurate wheel vibration measurement data, andmay provide multi-dimensional data as well, providing further basis foran accurate detection of wheel imbalance position upon the wheel as wellas of imbalance type.

In another embodiment of the system according to the present invention,the system is arranged to communicate with a data system in said vehiclefor a mutual exchange of data. Such data may advantageously comprisewheel radius data, vehicle speed indication, ABS system data provided tothe system and for the vehicle data system it may comprise a wheelimbalance indication signal, which is processed and communicated to adisplay system already available inside said vehicle, such as e.g. adisplay showing alert or alarm conditions in said vehicle.

By the invention it has been realized, that said method for detectingwheel imbalances may also be used to indicate periodic wheel vibrationsfrom a tire approaching its flat state.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described with reference to theaccompanying drawings, in which:

FIG. 1 a to 1 d are schematic side views of wheels in four differentwheel imbalance conditions, these being an uneven wear of a tire in FIG.1 a, wheel shape deformation from a circular one to an elliptical one inFIG. 1 b, an eccentric rotational axis of a wheel in FIG. 1 c, and afaulty suspension in FIG. 1 d;

FIG. 2 is a schematic side view of a vehicle comprising a systemaccording to one embodiment of the present invention;

FIG. 3 a to 3 d are schematic views of a vehicle rotation sensor signalfrom a system according to another embodiment of the present inventionin FIG. 3 a; and corresponding vehicle vibration sensor signals from anout of balance wheel as depicted in FIG. 1 a to 1 d in FIG. 3 b to 3 e,respectively; and

FIG. 4 is a graph showing a frequency response in a wheel imbalancedetection method according to one embodiment of the invention.

DETAILED DISCLOSURE

The FIGS. 1 a to 1 d were discussed in the above part of thedescription, and are used for illustration and as a reference for thethree dimensional coordinate system only.

In FIG. 2 is shown a vehicle 2 provided with a system 1 according to oneembodiment of the present invention for performing the method accordingto the invention of determining imbalances of at least one wheel 20 onsaid vehicle 2, when said wheel 20 is rotating on a surface G or islifted from such surface. Generally, said system 1 comprises a wheelvibration sensor 10 providing vibration signals and a wheel rotationsensor 12 providing wheel angular velocity signals including a referencesignal indicating the start of a wheel revolution for said wheel 20,where this in FIG. 2 is the front left wheel. Both sensors arecommunicating with a control unit 14 comprising a processor, which basedupon said signals performs a determination of whether an imbalancecondition exists for the wheel in question and in such case where uponthe wheel said wheel imbalance is located. The control unit 14 is incommunication with a display 16 indicating to a driver of the vehicle,what type of imbalance condition is detected upon which wheel, and whereupon such wheel it is located. Alternatively, the indicator means may bean indicator lamp or even a connection to a memory for later processingof the results from the vibration analysis, and/or display tomaintenance workers. Said indicator means may be provided inside thevehicle driving compartment or truck cab or outside the vehicle, or maybe provided as a data link to external maintenance surveillance systems.

The wheel vibration sensor 10 for said wheel 20 comprise in theembodiment shown in FIG. 2 three accelerometers (not shown) forobtaining a vibration signal along the x, y, and z direction, which arearranged on the inside of and adjacent to the wheel 20, which is mountedupon an axle (not shown) extending substantially along the z-direction.Preferably, the wheel vibration sensor 10 is provided on a non-rotatingpart of the axle for reducing vibrations deriving from such rotation ofthe axle. Accelerometers come in many varieties, includingpiezoelectric, potentiometric, reductive, strain gauge, piezoresistive,capacitive, and vibrating element accelerometers, which all share thecharacteristic of measuring a force in a given direction. Commerciallyavailable accelerometers may be delivered having one or moreaccelerometers in one unit measuring in the x-, y- and/or z-directionand have a size suitable for mounting inside small spaces, and may evenbe provided with communication means for delivering data with or withoutwires. When using an accelerometer measuring accelerations in more thanone dimension it is possible to accurately distinguish between a run-outand an out of balance type wheel imbalance, and also use these resultsto distinguish between other types of wheel imbalances by calculatingthe cross correlation and/or the phase relation between the longitudinaland vertical acceleration. Said wheel vibration sensor may be wheel hubmountable.

The wheel angular velocity sensor 12 provides signals corresponding tothe angular velocity ω of said at least one wheel 20. The angularvelocity sensor is preferably an ABS-sensor (Automatic Braking System),which provide a known number of pulses per wheel revolution. Asindicated in FIG. 3 a, said ABS-sensor is arranged to deliver areference signal 32 having a slightly longer pulse width than the otherpulses 30 for an indication of a known wheel revolution start 3.Typically, there are in the order of 60 pulses per revolution, and FIG.3 a is only serving illustrative purposes, not being accurate as tonumber of pulses per revolution. Separate revolutions are shown withsuch given revolution starting point 3.

As a further alternative, a less accurate angular velocity of the wheelmay be calculated based on signals from the available vehicle speedindicator and from data concerning the wheel radius r. In a preferredembodiment, the wheel radius data is supplied from a wheel data chip 12a provided in the tires upon the truck 2 by the time of tirefabrication. Alternatively, the radius r of the wheel 20 may beapproximated, or even be input by an operator of the system 1, e.g.maintenance personnel or the driver, or may be indicated to the system 1in any other suitable way 12. Alternatively, the start of wheelrevolution reference signal is given at the position of said wheelradius chip.

As shown in FIG. 2, these sensors 10, 12, 12 a communicate with thecontrol unit 14, which comprises a processor and a memory for acontinual signal processing of the measurements arriving from theaccelerometer 10 and the wheel speed sensor 12. Based on said signalprocessing, the control unit 14 is arranged to communicate the followinginformation to a driver of the truck 2: type of wheel imbalance andposition upon the wheel of such imbalance for each wheel provided withsensors 10, 12. An indicator or display 16 inside the truck cab isdisplaying this information, e.g. by displaying an image of the truckwith all wheels displayed as circular symbols along which is indicatedwhich type of imbalance is located where, e.g. in relation to groundcontact point. Alternatively, any results of the wheel imbalancedetection may be stored in a memory, which preferably may be provided inthe control unit 14. Thus, the indication is performed when e.g. amaintenance worker at the next service check inspects the content ofsuch a memory. The control unit 14 and the memory may be provided as oneunit, e.g. as a microcontroller or embedded system, reducing system sizesuitable for small spaces. Preferably, accelerometers 10 and angularvelocity sensors 12 are provided for all operative wheels for individualdetection of wheel imbalances, reducing service time significantly.

Preferably, the seriousness of such detected imbalance may also beindicated to a driver by actuating a specific visual or audible signaldevice or indicator lamp in said display. Further, the indicator 16 canalso be arranged to show only certain types of wheel imbalances, e.g. bya user selection option.

In the FIGS. 3 b to 3 e are shown examples of vibration signals overtime communicated to the control unit 14 from the wheel vibration sensor10 corresponding to a position upon the wheel 20 as indicated in FIG. 3a by the wheel angular velocity signals received from the rotationsensor 12, where the start of the revolution is at 3.

In FIG. 3 b is shown a periodic vibration signal or acceleration in they-direction as may be measured by the accelerometer from a wheel havingwheel imbalances as shown in FIG. 1 a, resulting in a discrete part of asine signal per revolution.

In FIG. 3 c is shown a periodic vibration signal or acceleration in they-direction as may be measured by the accelerometer from a wheel havingwheel imbalances as shown in FIG. 1 b, resulting in two continuouscomplete sine signals per revolution.

In FIG. 3 d is shown a periodic vibration signal or acceleration in they-direction as may be measured by the accelerometer from a wheel havingwheel imbalances as shown in FIG. 1 c, resulting in one continuouscomplete sine signals per revolution.

In FIG. 3 e is shown a periodic vibration signal or acceleration in they-direction as may be measured by the accelerometer from a wheel havingwheel imbalances as shown in FIG. 1 d, resulting in one or moreirregular non-sinoidal signals per revolution.

Accordingly, signals are available for signal processing for an accuratedetermination of wheel imbalance type and position upon said wheel, aswill be described further in the following.

The signal processing of the resulting wheel vibration measurements,which is performed in order to detect a frequency peak, which indicatesa periodic signal within a given frequency area, may be performed byusing a whole range of different analogue or digital techniques. Thesecomprise band pass filtering to reduce the signal to noise ratio duringtime domain analysis, Fast Fourier Transform or FFT and frequency domainanalysis, and may be performed either with predetermined or adaptivepeak detection levels. Preferably, the signal processing is performed bydigital frequency analysis of the signals acquired from the wheelangular velocity and vibration measurements.

For frequency domain analysis, the wheel vibration measurementsregistered by the accelerometer 10 at the wheel 20 may be represented asshown in FIG. 4, where the x-axis depicts the frequency and the y-axisdepicts the amplitude of the signals registered by the accelerometerover a series of measurements, e.g. a number of whole wheel revolutions,said number being large enough to provide a good statistical basis.Given that wheel imbalances tend to develop more slowly than any suchnumber of revolutions needed, this imposes no added risk to drivingsafety.

In FIG. 4 is shown the peak of the vehicular speed dependent 1st orderharmonic frequency fi, which is equal to the angular velocity ω at theperiphery of the wheel 20 divided by the wheel radius r. The 2nd orderharmonic f2 is shown as well. The detection level x or trigger level isindicated above, at which it is determined that an imbalance frequencypeak is detected. No peaks corresponding to any wheel imbalances areindicated in this figure. By subtraction of such a known frequencyspectrum, which may be determined by measuring over time adaptively orpredetermined as an approximation as a constant frequency spectrum beingdependent upon vehicle speed, any other periodic signals inside givenfrequency detection intervals and above the predetermined amplitudelevel may be determined as being indicative of wheel imbalance types,depending on number and frequency position of such overlaid periodicsignals.

Then, during frequency analysis, band pass filtering may preferably beperformed by selecting an interval ft−δf, ft+δf around one or more suchfrequencies, where such imbalance frequency peaks are presumed to belocated, where the value of δf may be suitably chosen to fit a peak mosteffectively. Each speed dependent peak frequency fi, f2, f3 . . . isequal to frequency of the wheel imbalance type and position on thewheel. The surrounding frequency level of said interval ft−δf, ft+δf isalso measured, and the peak level of the specific frequency is dividedby the surrounding frequencies ft−δf, ft+δf to be able to detect, if theincrease detected is a general noise increase or if the vibration signalis created by any wheel imbalance type, i.e. if ft/(ft−δf) orft/(ft+δf)>x, where x is a predetermined level, the result of thedetection wheel imbalance is positive. The detection amplitude thresholdlevel x may be chosen arbitrarily, depending upon type of indicationneeded, i.e. direct in the cabin provided for the driver attention or asservice data for maintenance personnel. Further, the level x may besubdivided into stages, for an indication of seriousness of theimbalance present for detecting the change of intensity of the detectedperiodic signal peak over time. Advantageously, type of indicator meansis selected according to different levels of seriousness of the wheelimbalance vibration.

As an example, consider a periodic vibration signal as shown in FIG. 3 bbeing induced into the vehicle wheel 20 due to the fact that the wheelhas developed an imbalance as shown in FIG. 1 a. Since the vibrationsignal as shown in FIG. 3 b is one discrete sinusoidal impulse per wheelrevolution having a lower frequency than the wheel revolution has, thefrequency peak should be clearly detectable in the lower frequency areabelow fi, by performing the above mentioned band pass filtering aroundthis frequency peak. A positive peak detection by said control unit 14combined with the processor correlating the peak detection in the timedomain with the position upon the wheel at that given time, see FIG. 3a, results in the processor being able to indicate the precise positionof the imbalance 20 a upon the wheel as well as the type of imbalancebeing detected, i.e. out of balance. In the case of a vibration signalhaving the form indicated in FIG. 3 c for a non-circular or ellipticalwheel shown in FIG. 1 b the processor is preferably able to indicatewhere upon the wheel 20 the maximum or minimum non-circularity 20 b isfound. For the signal shown in FIG. 3 d, the processor is able toindicate that the imbalance is located in the axial eccentricity and howlarge such eccentricity is. As shown in FIG. 3 e, no periodical signalis detected, and thus no indication of wheel imbalance is directlyvisible. However, the wheel imbalance is detectable by a general noiseincrease, which may be presented as part of the wheel imbalancecharacteristic as well.

The result from the previous quote is stored in the memory and comparedto the result of the next analysis. Preferably, overtone analysis isperformed for higher order harmonics as well in order to reconstruct anysuch weaker periodic wave signals. In general peak frequencies in theorder of 5 to 100 Hz are observed with the types of periodic wheelimbalances mentioned above.

The processor may preferably also be capable of indicating an informedsuggestion as to number, weight and position of required counterbalancing weights, when the wheel imbalance type is determined to be anout of balance type. This may render a second maintenance shop balancingapparatus unnecessary for balancing out an out of balance wheel.

The results of the digital signal processing may be stored continuouslyfor further processing or registration purposes, and may be interchangedwith an on board vehicle data system.

Other embodiments of the present invention are conceivable, allremaining within the scope of invention, such as the vehicle maypreferably be a truck, alternatively it may be an automobile, a bus or aconstruction vehicle, as well as a vehicle comprising trailers or atractor with a semi-trailer.

1. A method of determining type of imbalances of at least one wheel on avehicle, when the at least one wheel is rotating, comprising steps of:providing a vibration signal from at least one wheel vibration sensorassociated with the at least one wheel, the vibration signal comprisinga signal component indicative of at least vertical acceleration along ay-direction; providing angular velocity signals indicative of rotationof the at least one wheel, the angular velocity signals comprising areference signal indicating a start of a wheel revolution of the atleast one wheel; performing signal processing upon the vibration andangular velocity signals for detecting a periodic signal of apredetermined nature corresponding to imbalances on the at least onewheel and determining a position upon the at least one wheel of suchimbalance; and determining from the signal processing one or morecharacteristics of the imbalance indicative of imbalance type.
 2. Amethod according to claim 1, further comprising a step of: indicatingthe position of such imbalance on the wheel.
 3. A method according toclaim 1, wherein the angular velocity signals comprises a predeterminedseries of pulses, wherein one of the pulses is selected for indicating astart of a corresponding wheel revolution.
 4. A method according toclaim 1, wherein the reference signal is provided by an ABS sensorproviding a series of pulses per revolution, wherein at least one signalpulse in the series is different from other signal pulses therein forindicating a start of a corresponding wheel revolution.
 5. A methodaccording to claim 1, further comprising a step of indicating number,weight and position of counter balancing weights required to balance theat least one wheel, when the wheel imbalance type is determined to beout of balance.
 6. A method according to claim 1, wherein the signalprocessing comprises analogue and/or digital filtering performed byfrequency or time domain analysis.
 7. A method according to claim 6,wherein the frequency analysis comprises band pass signal filteringaround a selected frequency peak for determining an imbalance beingpresent when an amplitude of a filtered signal thereby generated isabove a predetermined amplitude detection threshold level x.
 8. A methodaccording to claim 7, wherein the signal processing step furthercomprises determining a degree of severity of the detected wheelimbalance by detecting an amplitude increase over time by dividing theamplitude detection threshold level into several stages.
 9. A methodaccording to claim 1, wherein the vibration signal is provided from atleast one-, two-, or three-dimensional accelerometer, which is providedon a non-rotating end section of an axle mounting the at least onewheel, adjacent to the at least one wheel.
 10. A method according toclaim 1, wherein the indication of the position of such imbalance in theat least one wheel, and optionally other wheel imbalancecharacteristics, such as imbalance type, is provided to maintenancepersonnel and/or to the driver of the vehicle.
 11. A system fordetermining type of imbalances of at least one wheel on a vehicle, whenthe at least one wheel is rotating, the system comprising: at least onewheel vibration sensor (10) associated with the wheel (20) operable toprovide a vibration signal comprising at least a signal componentindicative of vertical accelerations along a y-direction to a processor;at least one wheel rotation sensor (12) associated with the at least onewheel (20) operable to provide angular velocity signals to theprocessor; and a control unit (14) being operable to perform signalprocessing upon these signals for detecting a periodic signal of apredetermined nature corresponding to imbalances in the wheel; andindicator means (16) for indicating wheel characteristics beingdetermined for the at least one wheel by the signal processing; whereinthe at least one wheel rotation sensor further is arranged to provide areference signal (32) indicating a start of a wheel revolution to thecontrol unit (14) for a determination of the position on the wheel ofsuch wheel imbalance, when a positive detection is made; the controlunit (14) is operable to determine imbalance type from these signals;and indicator means operable to provide an indication such imbalancetype and imbalance position upon the at least one wheel.
 12. A systemaccording to claim 11, wherein the wheel rotation sensor is arranged toprovide a predetermined number of pulses, where one of the pulses isselected as the reference signal for providing an indication of a startof a wheel revolution.
 13. A system according to claim 11, wherein theat least one wheel rotation sensor is an ABS-sensor being provided withone tooth having a different width than other teeth of the sensor forproviding such reference signal.
 14. A system according to claim 11,wherein the at least one wheel rotation sensor is an angular velocitysensor being provided adjacent to the at least one wheel, and furthercomprising a start of one wheel revolution indicator provided on aperiphery of the wheel for providing the reference signal.
 15. A systemaccording to claim 11, wherein the at least one wheel rotation sensor isa combination of a wheel speed indicator, such as the on board vehiclespeed indicator, and a wheel radius indicator, such as a wheel data chipin the at least one wheel, and wherein the processor is operable tocalculate the angular velocity from an indication of vehicle speedprovided by the wheel speed indicator divided by a radius of the atleast one wheel, the wheel radius provided by the wheel radiusindicator, and the position of the wheel radius indicator providing thereference signal.
 16. A system according to claim 11, wherein the atleast one wheel vibration sensor (10) comprises at least a one-, two, orthree-dimensional accelerometer, which is provided on a non-rotating endsection of an axle mounting the at least one wheel, adjacent to the atleast one wheel.
 17. A system according to claim 11, wherein the atleast one wheel vibration sensor is provided upon an inner side of theat least one wheel.
 18. A system according to claim 11, wherein the atleast one wheel vibration sensor is wheel hub mountable.
 19. A systemaccording to claim 11, wherein the indicator means is accessible tomaintenance personnel and/or to a driver of the vehicle.
 20. A systemaccording to claim 11, wherein the control unit and indicator means arealso operable to determine and indicate, respectively, weight andposition of counter balancing weights required to balance the at leastone wheel when the wheel imbalance condition is an out of balance type.